1. Field of the Invention
This invention relates to a dichromatic mirror (otherwise known as an optical filter) in the form of a coated substrate. The term "dichromatic mirror" is used herein for a mirror which reflects a certain range of wavelengths and transmits another range of wavelengths. The present invention relates in particular to a dichromatic mirror which has a high reflectivity across most or all of the visible spectrum and a high luminous transmission over a narrow band of the visible spectrum. The invention includes within its scope a method for formation of the dichromatic mirror.
2. Description of the Related Art
While dichromatic mirrors may simply be employed for decorative purposes, for example in furniture, a major application is in road vehicle mirrors and especially in a vehicle rear-view mirror which is provided with a display light or detector positioned behind the mirror glass. Vehicle mirrors are necessarily located at positions which give a clear view to the rear of the vehicle, thus providing for any display light within the mirror to be clearly visible to other road users behind the vehicle. In particular the display light can usefully be a warning light, for example for brakes, hazard or turn signals.
Warning lights on vehicles are typically required to be red or amber. To incorporate such lights behind a vehicle mirror glass, it is necessary for the mirror glass to be transparent to the wavelength of the warning light, while retaining a high reflectivity without unacceptable coloration of the reflected image. Local regulations may require that the reflectivity of the mirror glass is at least 35%. In practice the reflectivity of an exterior mirror is typically from 40% to 70% and of an interior mirror may be as high as 70 to 80%.
U.S. Pat. No. 5,014,167 describes and claims a combination rear view mirror and brake or directional signalling lamp for a vehicle. The mirror is a semi-transparent dichroic (dichromatic) mirror combined with lighting means which operates as the brake lamp or directional signalling lamp. The lighting means is energised when the vehicle operator steps on the brake pedal or uses the directional signalling switch. The combination includes a baffle assembly which permits rearward escape of light from the lighting means but blocks light from travelling along the vehicle operator's line of sight.
European patent specification EP-A-0725286 relates to an optical filter "which is suitable for use in the mirror of a signalling device of the type described for example in the above-discussed U.S. Pat. No. 5,014,167". It describes its optical filter as a long wavelength pass filter (LWP) and claims a filter composite comprising a transparent substrate, an LWP thin film interference filter having a selected response in the visible spectrum, and selective absorbent means for providing low short wavelength transmittance, visual and ultra-violet opacity, and neutral reflectance. The LWP thin film interference filter may be a stack of alternating layers of high refractive index material and low refractive index material, which provides high transmittance in the red portion of the spectrum, and the high refractive index material absorbing radiation of wavelengths shorter than 600 nm. The high refractive index material may be selected from iron oxide, silicon, or certain metal sulphides, selenides, tellurides, or nitrides. The low refractive index material may be selected from certain metal oxides, certain metal fluorides, silicon oxide or silicon nitride.